GB2189086A - Camshaft drive - Google Patents

Abstract

A camshaft comprises a rotatable cam 10 member carrying an eccentric cam 12, a rotatable drive member 14 coaxial with the cam member 10 and defining with the cam member a space 16 filled with an ER fluid, this being a fluid of which the viscosity can be controlled by the application of an electric field. Means are also provided for applying an electric field to the ER fluid to control the viscosity of the fluid during rotation of the drive member so as to selectively couple and decouple the drive between the drive member and the cam member. <IMAGE>

Description

SPECIFICATION Camshaft drive The present invention relates to camshafts.

According to the present invention, there is provided a camshaft comprising a rotatable cam member carrying an an eccentric cam, a rotatable drive member coaxial with the cam member and defining with the cam member a space filled with an ER fluid, and means for applying an electric field to the ER fluid to control the viscosity of the fluid during rotation of the drive member so as to selectively couple and decouple the drive between the drive member and the cam member.

Electro-rheology is a phenomenon whereby the viscosity of certain materials (herein termed ER fluids) can be reversibly changed by the application of an electric field. Various fluids have been discovered exhibiting such properties one example being polydimethylsiloxene with added starch. ER fluids have already been developed which change rapidly from a liquid to virtually a solid. The properties of ER fluids can be tailored to meet the needs of particular applications and it is possible to achieve changes in viscosity from 1 to 10,000 centistokes. The voltages required to bring about such changes are high, for example 10 kV, but the currents are very low leading to a power consumption only of the order of a few milliwatts.

Switching can be fast with changes taking times of the order to only 0.1 msecs.

The present invention is predicated upon the realisation that such ER fluids lend themselves particularly well to several applications where it is desired to be able to selectively engage and a disengage the coupling between a cam and its drive.

The invention finds particular application in the automotive field and among these applications are the possibility of achieving the following programmable variable valve train functions, namely: a. Variable phase change and variable valve overlap, b. Valve de-activation and cylinder disablement, c. Variable early and late valve closing, and d. Variable valve period and variable valve lift.

The invention can also find application in fuel injection pumps to control the delivered quantity and the injection timing.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section through camshaft, and Figure 2 is an axial section through the camshaft of Figure 1 in the direction of the arrows A-A.

The illustrated camshaft comprises a short sleeve 10 carrying the cam profile 12 rotatably supported in a position to interact with the valve train but maintained in electrical isolation from the engine block, which acts as the electrical earth. A drive shaft 14 passes through the cam sleeve 10 leaving a small gap 16 which is closed at its ends by seals 18 to define a space filled with an ER fluid. The space communicates through passages in the drive shaft 14 with a reservoir of ER fluid which is not shown.

In illustrated embodiment, the sleeve 10 is journalled by means of first bearings 20 while the shaft 14 is journalled by means of bearings 22. It may be possibleto dispense with the bearings for the sleeve 10 if the ER fluid has sufficient compression strength, relying on the seals 18 for temporary support.

The sleeve 10 has a circular collar 30 which serves as a slip ring and contacts a brush 32 through which a high voltage can be applied across the ER fluid.

The collar 30 also includes a recess 34 which is engaged by a spring biassed latch 36 which acts as a detent to bring the sleeve 10 to rest in a predetermined position when the drive to it is disengaged. The latch may alternatively be arranged another part of the valve train such as on the cam 12, on the cam follower or on the stem of the valve. The purpose of the latch is to ensure that the cam remains stationary when not energised and also to ensure that the cam both stops at a known phase and recommences movement from the same phase. The spring in the latch should be sufficiently strong to bring the cam to rest by overcoming the inertial and frictional forces acting on the cam sleeve after it has been de-energised.

In the inactive mode, no voltage is applied across the ER fluid. The drive shaft 14 will rotate at constant speed in synchronism with the engine crankshaft, while the cam sleeve 10 remains stationary and slips on the drive shaft 14. At any time when a valve actuation is required, the ER fluid is energised by the application of a high voltage to render the fluid viscous and drive the cam 12 through a desired angle whereupon the ER fluid is de-energised and the cam is allowed to return to its base position.

To operate an engine cycle, the intake and exhaust valves must be on separate sleeves and independently switched at their respective times.

For a multi-cylinder engine, all the cams are electrically isolated and installed along the same drive shaft which acts as a common earth. By suitable electronic control means making use of the input information of the angular position of the engine crankshaft and the identification of the engine cylinder number, the ER cam system can be programmed to coordinate switching of all the valves according to the engine firing order and perform additional functions of introducing phase shift and/or valve de-activation individually to each valve as required.

The system operation is therefore analogous to an electronically controlled fuel injection system with the ER cams taking the place of the fuel injectors.

More specifically, the electronic control provides a fully programmable capability in the scheduling of the cam operation according to engine speed, load and drivability requirements.

The input drive shaft is driven by the engine crankshaft at a predetermined speed ratio which can be any ratio between a minimum of half the engine speed as in a conventional drive and up to a maximum ratio which corresponds to one complete cam sleeve rotation within the time required to produce the valve opening period.

For example, for a valve event period of 240 crank degrees, the maximum camshaft speed is that at which the crankshaft rotates through 240 degrees while the camshaft rotates through 360 degrees, this being a speed ratio of 3:2.

The cam profile on the cam sleeve should have the appropriate angular period to match the camshaft speed in order to produce the desired valve period in crank angle time. In the example above, at one and a half times crankshaft speed, the cam period is exactly 360 cam degrees (i.e. one complete rotation of the cam). In general, it is desirableto drive the camshaft at a high speed ratio which has the effect of giving a wide range of variable phase shift control.

For the minimum speed ratio of 1:2, the in-cycle decoupled period is zero and the cam phasing can only be altered in the retard direction whilst advancing direction cannot be achieved without having to deactivate the cam for one complete engine cycle.

By contrast, for a speed ratio of 3:2 as in the previous example, the in-cycle decoupled period is very long (i.e. 2 camshaft rotations) giving a very wide range of flexible control in both advancing and retarding the cam within each and every cycle.

The cam profile in this situation is also extremely desirable because of its large angular period which gives very generous radii of curvature and the high rotational speed ensures good oil film lubrication.

The applications of the programmable camshaft which were mentioned previously will now each be considered separately but it should perhaps first be mentioned that in all the applications the valve trains can be either spring-returned or desmodromic. In particular, the ER cam makes possible a simple form of desmodromic system using a single circular eccentric, this having many advantages over the conventional double cam desmodromic system.

Variable cam phase timing and variable valve overlap This is the basic control function of the ER cam.

The intake valve period can be phase shifted to optimise engine cylinder charging at all engine speeds to produce a high andflatfull load torque curve. The exhaust valve period can be phase shifted in the opposite direction to the intake valve phase shift to alter the valve overlap period which can be optimised for lower emissions and smooth idle/light load operation.

Valve de-activation and cylinder disablement For engines with more than one intake valve or intake port per cylinder, where the operation strategy is to de-activate one of the intakes during low air flow conditions in order to increase the charge velocity through the other operating intake, the ER cam offers the instantaneous selection of valve de-activation by simply not energising the ER fluid.

Ine 2 x 4 or 3 x 6 engine concept, which operates according to a strategy of selective cylinder disablement by switching off a bank of cylinders during idle and part load operations in order to increase the load factor of the remaining operating bank of cylinders and reduce the pumping losses associated with reduced throttling to the working cylinders, the ER cam can be programmed to disable any engine cylinder by simply deactivating both the intake and the exhaust valves.

One strategy is to disable predetermined banks of cylinders to select the number of working cylinders in fixed stages, for example 2 x 4,3 x 6, 2 x 4 x 6 etc. An alternative strategy is to follow the engine cylinder firing order and deactivate one cylinder at a time in a sequential factorial mannerto miss out one in a predetermined number of firings so that the power modulation is achieved in fine discrete steps with all cylinders having equal chance of firing.

Variable early and late intake valve closing (EIVC/ LIVC) These are strategies of power modulation to avoid throttling of the airflow in order to reduce pumping losses. Another application is to produce a variant of the Atkinson cycle by reducing the effective compression ratio by either EIVC or LIVC, and maintaining a high expansion ratio by a fixed high mechanical crankvolume ratio. The ER cam can produce EIVC and LIVC by decoupling the cam earlier than its usual one complete rotation period.

Such strategies should ideally be implemented with desmodromic valve trains with a moderately loaded spring return in preference to the conventional high speed spring-returned valve trains. By decoupling the cam before peak lift is reached, the valve spring return action will turn the cam lobe backwards and stop at the base position earlier than normal closing timing (there is also a reduction in the valve lift with this mode of operation). By decoupling the cam after peak lift is reached, the weak spring takes over the valve return function from the now freed desmodromic cam profile and returns the valve to the base position later than the normal closing timing. There is no free play throughout this operation as the valve remains fully constrained by the desmodromic system which limits its closing velocity as it approaches the valve seat.

Variable valve period and variable valve lift By a programmed combination of variable phase shift and variable early or late valve closing, the ER cam can produce a range of variable valve period changes which again can be optimised to suit different engine speed and load conditions.

By introducing another ER element to grip the valve stem, it is possible to hold a partial valve lift over an extended valve period which is in fact part load control by intake valve throttling. This has advantages over the previous methods of EIVC and LIVC in that high mixture turbulence is maintained throughout the intake stroke and the effective compression ratio is not impaired. Coordinated timing control of the ER cam and the ER valve elements is programmed by the controller to lift the valve to the desired lift by the ER cam, then decouple the cam but grip the valve stem till the time of the desired closing at which point the ER valve element is decoupled to allow the valve to close the return action of the spring.

Single eccentric desmodromic valve train The ER cam makes possible a simple and effective desmodromic valve train based on a single circular cam element. The follower is also a single element with two parallel guides to straddle the eccentric.

The valve action is sinusoidal with the opening and closing action executed in one sweeping action and with very gentle opening acceleration and closing deceleration.

Such an eccentric profile corresponds to a circular cam of 360 degrees angular period and the camshaft should correspondingly be driven at the maximum speed ratio as explained earlier to produce the desired valve period in one complete revolution.

Fuel-injection The system concept for this application is identical to the ER cam driven valve train where the actuation of the valve is replaced by the actuation of the plunger of a mechanical fuel injector. The injection timing is coordinated by electronic control of the ER cam. The injection quantity is controlled by utilising an operation mode similar to that of early valve closing described above where the valve lift (in this case the pumping stroke) is variable by inducing a shorter ER cam rotation to produce less than full stroke.

Such a system would be most suitable for high pressure fuel injection directly into the combustion chamber for diesel and petrol applications. The electronic control provides a fully programmable capability in the scheduling of the fuel calibration according to engine speed, load and drivability requirements.

Claims (13)

1. A camshaft comprising a rotatable cam member carrying an eccentric cam, a rotatable drive member coaxial with the cam member and defining with the cam member a space filled with an ER fluid, and means for applying an electric field to the ER fluid to control the viscosity of the fluid during rotation of the drive member so as to selectively couple and decouple the drive between the drive member and the cam member.

2. A camshaft as claimed in claim 1, wherein the cam member is a cam sleeve surrounding a drive shaft which serves as the drive member and defining with the drive shaft an annular gap sealed at its axial ends by means of rotary seals and containing the ER fluid.

3. A camshaft as claimed in claim 2, wherein the sleeve is separately journalled on bearings concentric with the axis of the drive shaft and is not supported by the drive shaft.

4. A camshaft as claimed in claim 2, wherein the sleeve is supported radially on the drive shaft by means of the ER fluid and the end seals.

5. A camshaft as claimed in any preceding claim, wherein the ER fluid is contained in a separate reservoir and circulates through the gap between the drive member and the cam member.

6. A camshaft as claimed in any preceding claim, further comprising a latch serving to hold the cam member in a predetermined position when not coupled for movement with the drive member.

7. A camshaft as claimed in any preceding claim, wherein the means for applying an electric field across the ER fluid includes a control circuit incorporating a microprocessor and connected to slipping contacts on the drive member and the cam member, the cam member being electrically isolated from the drive member.

8. An internal combustion engine having intake valves and/or exhaust valves each of which is actuated by means of a camshaft as claimed in any preceding claim.

9. An internal combustion engine as claimed in claim 8, wherein the cams for the individual valves are additionally controlled in such manner as to provide any one of the following functions, namely: a. variable phase change and variable valve overlap, b. valve de-activation and cylinder disablement, c. variable early and late valve closing, and d. variable valve period and variable valve lift.

10. An internal combustion engine as claimed in claim 8 or 9, wherein the valves are desmodromically operated and are not provided with strong return springs.

11. An internal combustion engine as claimed in claim 10, wherein each valve is operated by a single circular cam acting both to open and to close the valve.

12. A fuel injection pump comprising injector plungers driven by means of cams on a camshaft as claimed any one of claims 1 to 7, the camshaft being electrically controlled to vary the delivered quantity and/orthe injection timing.

13. A camshaft constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.